(1) Field of the Invention
This invention relates to a method and apparatus of recycling scrap and is more particularly concerned with a plastic metering assembly and a method of feeding simultaneouly two different bulk densities of plastic material into a processing apparatus.
(2) Discussion of the Prior Art
In the past, mechanisms have been developed for simultaneously feeding scrap material of two different bulk densities into a processing machine, such as a plastic extruder. The prior art includes U.S. Pat. No. 3,797,702 which provides an internal auger which protrudes downwardly through a stationary feed tube and thence, through the bottom portion of a lower hopper, the auger simultaneously feeding aggregate material from an upper hopper and a lower hopper into a processing machine.
The prior art also includes U.S. Pat. No. 4,108,334 which discloses another type of apparatus for simultaneously feeding two densities of bulk material. The device of this patent includes a central rotatable shaft having rotary impellers and a side conduit which feeds sidewise into the stream of material fed by the main shaft. A second motor rotates, in the side conduit, a shaft provided with impellers for feeding this material into the main stream of material.
The prior art devices discussed above cannot achieve a high rate of speed in the feeding of plastic into the processing machine and do not accurately meter and vary the amount of low bulk density material fed to the extruder or other processing machine.
A search of the prior art also disclosed U.S. Pat. Nos. 1,268,813; 1,925,541; 2,517,456; 2,995,775; 3,148,412; 3,177,527; 3,182,969; 3,231,243; 3,386,707; 3,679,101; 3,871,629 and 4,014,462 which are believed to be less pertinent than the patents discussed above.
The significant difference between my scrap metering assembly and the prior art cited is that the scrap material (low bulk density) and the primary material (base or high bulk density) are kept separate from each other until both are forced simultaneously into the flights of the extruder screw of the extruder. The amount of scrap which may be reprocessed in an extruder is controlled first by the compression ratio of the extruder screw which defines the minimum bulk density of feed material which may be presented to it, and secondly the ability of the extruder screw to plasticize and homogenize the dissimilar mixture of materials presented to it (pellet and ground scrap). Most extruder screws are designed with a 2.5 to 3.0 compression ratio, this means that the volume of the extruder screw in the feed (unmelted) section is 2.5 to 3.0 times the volume in the metering (melted) section of the screw. For example, a typical extruder running low density polyethylene at a specific gravity of 0.928 with a screw having a compression ratio of 3.0 the minimum bulk density which may be introduced into the extruder screw feed section is 0.928.times.62.4/3.0=19.3 pounds per cubic foot. Since the bulk density of the primary (pelletized) plastic is approximately 35 pounds per cubic foot, it is apparent that the primary material may be blended with scrap material of a lower bulk density by continuing to add scrap material until the average bulk density reaches about 19.3 pounds per cubic foot. To continue to add scrap beyond this point would reduce the output of the extruder screw. The present invention achieves this blending to produce the desired density.